1. Field of the Invention
This invention relates to electrophotography. In particular, the invention relates to chemical sensitization of heterogeneous photoconductive compositions and electrophotographic elements with heteroaromatic compounds in which nitrogen is the sole hetero atom.
2. Discussion of Related Art
The process of xerography, as disclosed by Carlson in U.S. Pat. No. 2,297,691 (issued Oct. 6, 1942), employs an electrophotographic element comprising a support material bearing a coating of a normally insulating material whose electrical resistance varies with the amount of incident electromagnetic radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge, generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of this surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or discharge pattern as desired. Deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor, or the like, or transferred to a second element to which it can similarly be fixed. Likewise, the electrostatic latent image can be transferred to a second element and developed there.
Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support, and particles of photoconductive zinc oxide dispersed in resinous, film-forming binder have found wide application in the present-day document copying applications.
Since the introduction of electrophotography, a great many organic compounds have also been screened for their photoconductive properties. As a result, a very large number of organic compounds have been shown to possess some degree of photoconductivity. Many organic compounds have revealed a useful level of photoconduction and have been incorporated into photoconductive compositions.
In photoconductive insulating compositions using organic photoconductors, the photoconductor, if not polymeric, is usually carried in a film-forming binder. Typical binders are polymeric materials having fairly high dielectric strength such as phenolic resins, ketone resins, acrylic ester resins, polystyrenes and the like. A more comprehensive listing of binders appears in U.S. Pat. No. 3,755,310 (issued Aug. 28, 1973 to L. J. Rossi). The photoconductor can be dissolved with the binder to prepare a homogeneous photoconductive composition in a common solvent. Or, it can be provided as a dispersion of small particles in the binder to prepare a heterogeneous photoconductive composition. A general discussion of such dispersions and their preparation appears in U.S. Pat. No. 3,253,914 (issued May 31, 1966 to G. Schaum). There is no suggestion in the U.S. Pat. No. 3,253,914 of binders that are especially useful for preparing the dispersions.
Organic photoconductors demonstrate widely varying degrees of solubility in the organic solvents used to dissolve many of the common binders. In the preparation of homogeneous photoconductive insulating compositions, organic photoconductors such as p-terphenyl and others of low solubility in popular solvents cannot usually be included in sufficient concentration to provide compositions of desirable light-sensitivity. By use of dispersion techniques such as those used with zinc oxide photoconductors, heterogeneous photoconductive insulating compositions having higher concentrations of low solubility photoconductors can be obtained, the objective being to improve light-sensitivity in the composition.
Heterogeneous organic photoconductive compositions as discussed herein can be advantageous, especially in the preparation of electrophotographic elements on which visible images will be provided. For example such elements are both lighter in weight than those having inorganic photoconductors like zinc oxide, and can be prepared to resemble bond paper. However, in such applications they have not enjoyed the popularity of photoconductive insulating compositions using inorganic photoconductors. This is largely due to the unacceptable photoconductivity (or speed) of such heterogeneous compositions despite high concentrations of organic photoconductor. Homogeneous compositions of organic photoconductors, on the other hand, appear to be acceptable in their photoconductivity, but when coated on paper materials, they do not have the appearance and feel of plain paper.
To improve the photoconductivity of heterogeneous photoconductive compositions comprising dispersed organic photoconductor particles, a variety of compounds have been studied for use as so-called chemical sensitizers or activators. When added to the photoconductive compositions it is intended that such compounds enhance the photoconductivity of the composition at least within the electromagnetic wavelength region in which the composition is intrinsically sensitive. If successful, the composition is said to be chemically sensitized or activated. Owing to the commercial popularity of homogeneous photoconductive compositions, however, the art has, for the most part, taught chemical sensitizers which are specific to homogeneous compositions. In this regard, it does not generally follow that a chemical sensitizer that is useful in a homogeneous mode, is also useful in a heterogeneous mode. Also, as is the case in the present invention, the binder employed in a heterogeneous photoconductive insulating composition can affect not only the photoconductivity of the composition, (see U.S. Pat. No. 3,703,372, issued Nov. 21, 1972 to S. H. Merrill) but also can affect the ability of the composition to be chemically sensitized.
In U.S. Pat. Nos. 3,152,895 (issued Mar. 14, 1962 to G. H. Tinker et al) and 3,607,261 (issued Apr. 4, 1969 to A. B. Amidon) a variety of binders, including cellulose nitrate, are disclosed for use in photoconductive compositions comprising either inorganic or organic photoconductors. Neither of these patents, however, provide any specific teaching as to sensitization with compounds of the present invention nor any suggestion that the ability of a photoconductive composition to be sensitized may be affected by the binder employed.
N-heteroaromatic compounds of the type disclosed herein have been previously used in the field of electrophotography. See, for example, U.S. Pat. No. 3,279,918 issued Oct. 18, 1966 to P. M. Cassiers et al; British Pat. No. 1,289,529 published Sept. 20, 1972 issued to Canon Kabushiki Kaisha. These patents, however, neither point out nor suggest the use of such compounds with any particular binder to provide usefully sensitized heterogeneous compositions of dispersed organic photoconductor particles.
Selection of a proper chemical sensitizer is further complicated by other requirements of an electrophotographic process. An element employing a chemically sensitized photoconductive composition as defined herein must, for example, readily accept and hold electrostatic charge before imagewise illumination. Often compositions employing compounds screened for use as sensitizers, although acceptably photoconductive, fail to accept a sufficiently high charge to be useful in a practical sense. Compositions exhibiting such failure are said to be "charge saturated". Further, though able to accept charge, compositions may be unable to hold applied charge for reasonable periods of time in the dark hence the term "dark decay".